1. Field of the Invention
The present invention relates to a band elimination filter and a duplexer, and more particularly, to a band elimination filter and a duplexer for use in a radio communication apparatus and the like which operate in a microwave band and a millimeter wave band.
2. Description of the Related Art
Recently, as a filter for use in radio communication apparatus which operates in a microwave band and a millimeter wave band, there have been proposed a band elimination filter including a ring shaped resonator (see Japanese Examined Patent Publication No. 2516984), and a band-pass filter including a ring shaped resonator (see Electronic Information Communication Society Engineering Report (Japanese name: Denshi-joho-tsushin gakkai-giho), May, 1996).
The conventional band-pass filter resonators are adapted to resonate in two orthogonal modes combined together, namely, in a dual mode. However, the filters have a problem that their insertion loss is high, due to the fact that the filters are band-pass filters.
The ring shaped resonator used in the conventional band elimination filter aims at enhancing the attenuation factor in a passing-band of the band-pass filter. Moreover, the two orthogonal modes of resonance are independent from each other. This causes a problem that an input-output coupling capacitor, in addition to the ring shaped resonator, needs to be inserted between an input-output terminal and the ring shaped resonator for construction of the band elimination filter.